1. Field of the Invention
The present invention relates to a far infra-red heater. More specifically, the present invention pertains to a far infra-red heater which comprises a ceramic resistance heating element obtained by mixing and dispersing an insulating heat resistant component and a conductive component, wherein the resistance heating element per se directly radiates rays having a wave length falling within the far infra-red region efficiently.
2. Description of the Prior Art
Up to now, there have been knwon various far infra-red heaters such as (1) an infra-red heater comprising a quartz tube and a tungsten filament enclosed therein or a quartz tube heater comprising a quartz pipe and a nichrome wire enclosed therein; (2) a heater obtained by coating the metal surface of a sheathed wire heater, which comprises a metal tube and a nichrome wire enclosed therein through an insulater such as magnesium oxide, with a ceramic far infra-red radiating material such as those comprising alumina, zirconia and titania; and (3) a heater comprising a ceramic tube made of the foregoing far infra-red radiating material and a nichrome wire enclosed therein.
In the aforementioned far infra-red heaters, a commercial voltage is in general applied to both ends thereof to generate Joule heat whereby the temperature of the surface of the heater is raised to a predetermined level ranging from 200.degree. to 600.degree. C. If the temperature of the heater is raised, the radiant quantities of infra-red rays correspondingly increase, thus a substance to be heated is irradiated with the infra-red rays radiated by the heater and the surface of the substance absorbs the infra-red rays whereby the substances per se are heated.
For this reason, the heating effect of the far infra-red rays greatly depends on the radiation properties of a far infra-red heater and the infra-red absorption characteristics of a substance to be heated.
In other words, a far infra-red heater should radiate infra-red rays compatible with the absorption characteristics of a substance to be heated. Under such circumstances, various kinds of heaters have practically been used depending on a variety of applications. Examples of typical applications of such heaters are baking and drying of paints, inks or the like, drying of lumbers, grilling of foods and heating such as floor heating and a sauna.
The foregoing conventional far infra-red heaters suffer from the following disadvantages:
First of all, in the aforementioned infra-red lamp or the quartz tube heater (1), light generated by an electrically heated wire is radiated through quartz wall. As a result, the wave length of the principal radiant rays falls within the range of near infra-red rays in the order of 1.5.mu. and, therefore, such a heater or a lamp does not radiate sufficient quantity of light having a wave length falling within the far infra-red range. Moreover, these heaters have low mechanical strength.
Although the aforesaid heaters (2) obtained by coating a sheathed wire heater with a ceramic far infra-red radiating material efficiently radiate far infra-red rays having a wave length of 3 to 50.mu., they suffer from an inevitable problem that the ceramic radiating material is peeled off from the surface of the metal tube due to the difference between the thermal expansion coefficients of the metal tube and the ceramic radiating material applied onto the surface of the former.
The foregoing heaters (3) comprising a ceramic tube and a nichrome wire enclosed therein make it possible to solve the problem of peeling off of the coated material associated with the foregoing sheathed wire heaters (2), they can be made lighter since it is not necessary to use any insulating materials and they make it possible to improve their thermal efficiency. However, they still suffer from drawbacks originated from the fact that the heating is performed by an indirect heating method in which a radiant is indirectly heated by heating a nichrome wire. More specifically, a problem that the electrically heated wire such as a nichrome wire is locally heated abnormally to thus result in burning out of the wire due to the increase in the resistance of the wire because of its oxidation and corrosion has not yet been solved. Moreover, they further suffer from the problems concerning, for instance, thermal energy loss due to indirect heating; uneven distribution of temperature and retardation of response time in the temperature control.